1. Field of the Invention
The present invention relates to a 90xc2x0 phase shifter, and more specifically to a 90xc2x0 phase shifter for use in a direct conversion tuner used in a digital satellite broadcast receiver. The present invention relates also to a demodulator employing such a phase shifter.
2. Description of the Prior Art
In recent years, with the development of satellite communication techniques, digital satellite broadcasting has come into operation on the basis of such satellite communication techniques. In a receiver for digital satellite broadcasts, a direct conversion tuner is used that is provided with a demodulator that performs quadrature demodulation on a received signal to demodulate it into a baseband signal. Such a demodulator is provided with a phase shifter that outputs two signals separated in phase by 90xc2x0 from each other. Now, a conventional phase shifter will be described with reference to the drawings.
FIG. 7 shows a quadrature demodulator employing a conventional phase shifter. The quadrature demodulator shown in FIG. 7 has an input terminal 1 at which it receives a received signal, a mixer 2 that produces an I baseband signal from the received signal fed from the input terminal 1, a mixer 3 that produces a Q baseband signal from the received signal fed from the input terminal 1, a phase shifter that generates two oscillation signals separated in phase by 90xc2x0 from each other from a local oscillation signal fed from a local oscillator 5, a local oscillator 5, an output terminal 6A to which the I baseband signal is delivered from the mixer 2 for output, and an output terminal 6B to which the Q baseband signal is delivered from the mixer 3 for output.
In this demodulator configured as described above, the phase shifter 4 is, as shown in FIG. 8, composed of an all-pass filter 41 that produces from the local oscillation signal an oscillation signal separated in phase by 45xc2x0 therefrom and feeds the produced signal to the mixer 2 and an all-pass filter 42 that produces from the local oscillation signal an oscillation signal separated in phase by 135xc2x0 therefrom and feeds the produced signal to the mixer 3. Thus, through the operation of the all-pass filters 41 and 42 provided within the phase shifter 4, the phase shifter 4 outputs two oscillation signals that are used as two carriers separated in phase by 90xc2x0 from each other.
The all-pass filters 41 and 42 are each composed of circuit elements as shown in FIG. 9, specifically npn-type transistors Q1 and Q2, resistors RLA and RLB connected to the collector of the transistors Q1 and Q2 respectively, resistors RKA and RKB connected to the emitter of the transistors Q1 and Q2 respectively, capacitors CA and CB connected between the collector and base of the transistors Q1 and Q2 respectively, and a constant current source 43 connected to the node between the resistors RKA and RKB. In addition, a supply voltage VCC is applied to the node between the resistors RLA and RLB.
In the all-pass filter shown in FIG. 9, an input signal VIN fed between the bases of the transistors Q1 and Q2 causes an output signal VOUT to appear between the node connecting the collector of the transistor Q1 to the resistor RLA and the node connecting the collector of the transistor Q2 to the resistor RLB. Now, suppose that the resistors RKA, RKB, RLA, and RLB all has a resistance R, and that the capacitors CA and CB both has a capacitance C. Then, the gain Gv of this all-pass filter configured as described above is given by formula (1) below. Hence, the phase characteristic between the input signal VIN and the output signal VOUT is expressed by formula (2) below. It is to be noted that xcfx89 represents 2xcfx80f, assuming that the input signal VIN has a frequency of f.                     Gv        =                              (                          1              -                              j                ⁢                                  xe2x80x83                                ⁢                ω                ⁢                                  xe2x80x83                                ⁢                CR                                      )                                (                          1              +                              jω                ⁢                                  xe2x80x83                                ⁢                CR                                      )                                              (        1        )            xe2x80x83xcfx86=xe2x88x922 tanxe2x88x921(xcfx89CR)xe2x80x83xe2x80x83(2)
Since the phase characteristic of the all-pass filter is expressed by formula (2), the product CR of the resistance R and the capacitance C of the resistors and capacitors used in the all-pass filter is given by formula (3) below.                     CR        =                              tan            ⁡                          (                              -                                  φ                  2                                            )                                ω                                    (        3        )            
Hence, when a local oscillation signal having a frequency of 1 [GHz] is fed to the all-pass filter 41, which should yield an output signal separated in phase by 45xc2x0 from the input signal fed thereto, formula (3) requires that the value of CR be equal to 6.592xc3x9710xe2x88x9211. In this way, in the all-pass filter 41 to which a high-frequency local oscillation signal is fed, the product RC of the resistance R and the capacitance C of the resistors and capacitors used therein needs to be considerably small, and thus the resistance R and the capacitance C need to be accordingly small.
However, making the resistance R of the resistors smaller requires increasing the areas of the resistors, and is accompanied by an increased influence of the parasitic capacitance appearing in the resistive films of the resistors. On the other hand, making the capacitance C of the capacitors smaller makes the influence of the stray capacitance of the wiring pattern laid in the all-pass filter too great to ignore. As a result, when the phase shifter is formed in an integrated circuit, the higher the frequency of the signal it needs to handle, the lower its accuracy.
Moreover, in a demodulator employing such a phase shifter, the frequency of the received signal fed in via its input terminal is equal to the frequency of the local oscillation signal generated by the local oscillator provided therein. This causes the local oscillation signal to leak back to the input terminal by way of the signal, power, and ground lines. In addition, conversely, the received signal destabilizes the operation of the local oscillator that generates the local oscillation signal having the same frequency as the received signal, and thereby causes the frequency of the local oscillation signal to fluctuate, thus causing also the frequencies of the I baseband and Q baseband signals output from the demodulator to fluctuate.
An object of the present invention is to provide a phase shifter, and a demodulator employing it, that generates high-frequency local oscillation signals separated in phase by 90xc2x0 from each other.
Another object of the present invention is to make it possible to manufacture such a phase shifter with a high degree of integration.
To achieve the above object, according to one aspect of the present invention, a phase shifter is provided with: a phase shifting portion that produces, from a local oscillation signal having a frequency of f/n (where n is a natural number), two signals separated in phase by 90/n degrees from each other; and a frequency multiplying portion that performs frequency multiplication on each of the two signals output from the phase shifting portion by a frequency multiplication factor of n so that the frequency of those signals is converted from f/n to f and that makes those signals separated in phase by 90 degrees from each other.
According to another aspect of the present invention, a demodulator is provided with: a phase shifter including a phase shifting portion that produces, from a local oscillation signal having a frequency of f/n (where n is a natural number), two signals separated in phase by 90/n degrees from each other and a frequency multiplying portion that performs frequency multiplication on each of the two signals output from the phase shifting portion by a frequency multiplication factor of n so that the frequency of those signals is converted from f/n to f and that makes those signals separated in phase by 90 degrees from each other; a first mixer that produces an I baseband signal by multiplying a received signal fed in from outside by one of the two signals output from the phase shifter; and a second mixer that produces a Q baseband signal by multiplying the received signal fed in from outside by the other of the two signals output from the phase shifter.